Method for forming titanium sheets

ABSTRACT

A method of forming titanium sheets and the like in which a sandwich structure consisting of a titanium sheet is placed in an envelope constructed of mild hot rolled steel, forming the sandwich structure utilizing a stretch forming operation, bracing the sandwich structure insuring that it will retain its shape during a stress-relieving operation, performing the stress relieving operation and removing the shaped titanium sheet from the envelope.

Unite States Patent 1191 Manchester et al.

[ METHOD FOR FORMING TITANIUM SHEETS [75] Inventors: William E. Manchester, Lomita; Carl M. Morris, Manhattan Beach; John L. Hill, Los Angeles County,

all of Calif.

[73] Assignee: Northrop Corporation, Los Angeles,

Calif.

[22] Filed: Feb. 22, 1971 211 Appl. No.2 117,403

[52] US. Cl ..29/423, 29/DIG. 45, 72/700 51 1111. C1. ..B23p 17/00 [58] FieldofSearch ..29/423,424,DIG.45, 196, 29/191, 198; 72/364, 296, 378, 700, 379;

[56] References Cited UNITED STATES PATENTS 2,289,31 l 7/1942 Wellman ..29/423 X 1 51 Mar. 27, 1973 2,993,269 7/1961 Kelley .29/424 3,122,828 3/1964 Havd .29/423 3,236,086 2/1966 Steigerwalt ..72/296 3,262,301 7/1966 Langworthy .....72 296 3,339,271 9/1967 Durfee et 31.... ....,29/423 x 3,584,368 6/1971 Sargent, Jr. .29/424 Primary Examiner-John F. Campbell Assistant Examiner-Victor A. Di Palma Attorney-Harold L. Fox and Willard M. Graham [57] ABSTRACT A method of forming titanium sheets and the like in which a sandwich structure consisting of a titanium sheet is placed in an envelope constructed of mild hot rolled steel, forming the sandwich structure utilizing a stretch forming operation, bracing the sandwich structure insuring that it will retain its shape during a stress-relieving operation, performing the stress relieving operation and removing the shaped titanium sheet' from the envelope.

6 Claims, 5 Drawing Figures METHOD FOR FORMING TITANIUM SHEETS The invention pertains to a method of forming titanium sheets and the like and more particularly to a more economical method of forming and contouring titanium sheets than those presently known.

The sheets, referred to above, are designed for use in the outer surfaces of airplanes designed to fly at supersonic speeds. It is well known that the temperature of outer surfaces of an airplane, travelling at supersonic speeds, exceeds the temperatures at which aluminum is a satisfactory material and titanium alloys are a desirable material. Therefore, it is highly desirable that a more economical method of compound contour forming titanium sheets be developed than those currently in use.

Briefly the present invention relates to a method of forming titanium sheets in which a titanium sheet is secured between a pair of hot rolled steel sheets, securin g the titanium sheet between the steel sheets by welding or the like to provide a sandwich structure, contouring the sandwich structure utilizing a stretch forming operation. The contour formed in the sandwich is retained therein by placing a plurality of tie bars extending between the edges of the sandwich structure. The assembly is then stress relieved by heating the assembly at a predetermined temperature for a given period of time. The formed titanium sheet is then removed from the envelope and placed in storage for future use.

An object of the present invention is to disclose a method of forming titanium sheet which is more economical than the presently used vacuum creep or other known methods.

Another object is to disclose a method of forming titanium sheets which can be performed with equipment normally found in an airframe factory, using conventional labor practices used on aluminum airframe parts.

Although the characteristic features of the present invention are particularly pointed out in the appended claims, the invention itself, also the manner in which it may be carried out, will be better understood by referring to the following description taken in connection with the accompanying drawings forming a part of this application and in which:

FIG. 1 is a perspective view of a titanium sheet being formed by the method disclosed herein.

F IGS. 2 and 3 are sectional views as indicated by the arrows 2-2 and 33 of FIGS. 1 and 2, respectively.

FIG. 4 shows an intermediate step in forming of the titanium sheet as disclosed herein.

FIG. 5 is a perspective view of a formed titanium sheet formed by the method as disclosed herein.

Referring to the drawing and particularly to FIG. 5, a titanium sheet 11 formed by the method as disclosed herein is shown. Specifically the sheet 11, having a compound curve formed therein, is shaped to provide the outer covering or skin of a supersonic or hypersonic airplane. Because of its characteristics, from a weight and temperature standpoint, it provides a most satisfactory surface for airplanes referred to above and is intended to provide titanium sheets normally formed by vacuum creep or other methods.

Both sides of the sheet 11 are first cleaned by any conventional method and coated with an acceptable protective coating. The sheet 1 1 is then placed between two sheets 12 and 14 fabricated of mild steel, as shown in FIGS. 1, 2, 3 and 4. In this respect one of the sheets 12 or 14 must be joggled as indicated by the numeral 16 to receive the sheet 11. If the sheet 11 exceeds 0.150 inch in thickness perimeter strips 15 may be placed around the sheet 11. Normally the steelsheets are approximately two and one half to three (2 A to 3) times the thick-mess of the titanium sheets being formed, the sheet 11 is in the normal range from 0.030 inch to 0.135 inch thick.

The measurements of the depressed portion (joggle 16) in the steel sheets exceed those of the sheet 11 by at least 1 inch. The protective coating on the sheet 12 insures free relative movement between the sheets 11, 12 and 14 during the forming operation. The sheets 12 and 14 are then spot welded, as indicated by the numeral 21, around the perimeter of the sheet 11, the spot welds 21 are 1 V2 inch from the perimeter of the sheet 11. Thus a sandwich structure 17 or envelope consisting of the sheets 11, 12 and 14 is formed.

The sheet 11 and envelope 17 are formed or shaped utilizing a stretch forming apparatus for this operation. The sandwich structure 17 is formed on a stretch form die 18 having the same configuration as the surface configuration of the airplane that the sheet 11 is to cover. In this respect the sheets 12 and 14 must extend over the ends or sides sufficiently to provide gripping allowance for the stretching apparatus.

The sandwich 17 is firmly and securely held in the sheets 12 and 14 by the spot welds 21. The titanium sheet 11, however is free to slip within the plates 12 and 14 during the stretch forming operation, the stretching forces are applied primarily to the steel cover sheets or envelope 17. After the stretch forming operation is performed the sandwich structure 17 is removed from the forming or stretching apparatus.

The envelope or sandwich structure 17 is removed from the forming apparatus and steel tie bars 19 are welded between the edges of the structure, from edge to edge, substantially as shown in FIG. 4. This is to preserve the shape of the structure (sandwich 17) during handling and a subsequent stress relieving operation. With the tie bars 19 welded in position the structure (sandwich l7 is placed in a furnace where a stress relieving operation is accomplished. This consists of heating the structure (sandwich 17) to the stress relieving temperature for approximately 30 minutes.

After the stress relieving operation the structure (sandwich 17) is removed from the furnace, the spot welds 21 are removed by drilling at least on three sides of the sheet 11, and the sheet 11 placed in storage for future use. As a general rule the sandwich structure 17 should be stretched approximately 5 to 10 percent insuring that the proper contour of the sheet 1 1 has been attained.

Iclaim:

1. A method of forming a sheet of titanium which comprises the following steps:

a. enclosing a sheet of titanium between two sheets of steel providing a sandwich structure;

b. securing the sheet of titanium between the steel sheets by welding or the like;

c. forming the sandwich structure utilizing stretch forming apparatus;

(1. removing said sandwich structure from the stretch forming apparatus;

e. stress relieving the sandwich structure by heating forth in claim 4:

the latter for a predetermined period of time; a. in which said stress relieving operation constitutes f. and removing said titanium sheet from said sandheating said sandwich structure at the titanium wich structure. alloy stress relieving temperature for a specified 2. The method of forming a titanium sheet as set 5 length Oftimeforth in claim 1: 6. A rnethod of forming a sheet of titanium which a. in which each of said two steel plates are approxiprise the ollowing steps:

mately 2 to 3 times the thickness of said titanium enclosmg a sheet of tltanlum between tWQ Sheets sheet. of steel providing a sandwich structure; 3, Th h d f f i i i sheets as set f th b. securing the sheet of titanium between the steel in claim 2: sheets by welding or the like;

a. in which peripheral strips are secured to at least forming the sandwich structure utilizing a Stretch one of the steel sheets and are placed around the formmg apparatus; titanium Sheet d. fixedly securing a plurality of tie bars between the 4. The method of forming titanium sheets as set forth edges of Sald Sandwlch structure to retain its in claim 3: Shane;

a. in which a plurality of tie rods are welded between -lF Y sandwlch strglcture, Wlth sald tle the edges of said sandwich structure to insure the bays poslflonr rehevmg operatloni integrity of its Shape during a stress relieving f. and removing said titanium sheet from said sandoperation web structure. 5. The method of forming a titanium sheet as set 

2. The method of forming a titanium sheet as set forth in claim 1: a. in which each of said two steel plates are approximately 2 to 3 times the thickness of said titanium sheet.
 3. The method of forming titanium sheets as set forth in claim 2: a. in which peripheral strips are secured to at least one of the steel sheets and are placed around the titanium sheet.
 4. The method of forming titanium sheets as set forth in claim 3: a. in which a plurality of tie rods are welded between the edges of said sandwich structure to insure the integrity of its shape during a stress relieving operation.
 5. The method of forming a titanium sheet as set forth in claim 4: a. in which said stress relieving operation constitutes heating said sandwich structure at the titanium alloy stress relieving temperature for a specified length of time.
 6. A method of forming a sheet of titanium which comprises the following steps: a. enclosing a sheet of titanium between two sheets of steel providing a sandwich structure; b. securing the sheet of titanium between the steel sheets by welding or the like; c. forming the sandwich structure utilizing a stretch forming apparatus; d. fixedly securing a plurality of tie bars between the edges of said sandwich structure to retain its shape; e. subjecting the sandwich structure, with said tie bars in position, to a stress relieving operation; f. and removing said titanium sheet from said sandwich structure. 